This invention applies to the field of fiber optics illuminators, and more particular to those illuminators that provide a source of illumination for plastaic fiber bundle light guides.
A principal disadvantage of such currently known fiber optics illuminators is that they produce intense focussed light onto the receiving end of the fiber bundle, which tends to heat, age and often burn the surface of the fiber ends, destroying the light guide prematurely. The recommended maximum continuous operating temperature for commercial plastic optical fibers is 70.degree. C. the fiber ends soften, distort and begin to melt.
Many thermal control methods are presently used to reduce the heat to protect the end of the fiber bundles in state-of-the-art illuminators. One common method is the use of dichroic ellipsoidal reflector lamps, such as the "MR" (Miniature Reflector) halogen lamps that allow a substantial portion of the infrared energy from the lamp to pass through visible-reflectance dichroic glass reflectors instead of being reflected with the visible light into the focussed beam. Experiments by the inventor have shown that such MR lamps with power as low as 42 watts will melt plastic optical fibers at the focal point within 15 seconds.
Another additional method commonly used to reduce the heat in the beam is the use of an infrared reflecting dichroic mirror between the lamp and the fiber bundle. This reduces the visible energy by only about 10%, and has the effect of reducing the heat load so that the 42 watt bulb will melt the plastic fiber ends at the focal point in about 30 seconds.
Some presently used illuminators tilt an infrared-transmitting, visible-light-reflecting dichroic mirror at a 45.degree. angle to the optical axis, but since the mirror is near Brewster's angle, the visible beam energy is reduced by as much as 50%, and the reflected light is strongly polarized by the grazing reflection. The results is reduced thermal energy in the beam, but the technique usually only doubles the time to fiber melting to about 60 seconds.
Another method in common use for thermal control is the use of a relatively high velocity cooling fan to blow air across the end of the fiber bundle. This improves cooling, but such systems still do not preclude fiber burning at the focal point within a relatively short time.
After employing all of the foregoing heat removal methods, presently known fiber optics illuminators take the final step to prevent melting and burning of plastic fibers; they defocus the beam so only a portion of the energy in the visible beam strikes the fiber bundle. This technique is characteristic of all presently known illuminators that use 30 watt lamps or larger to illuminate plastic optical fibers. As a matter of fact, the manufacturers of plastic fibers provide technical manuals for their use which instruct the user not to place the fiber ends at the focus of the optical system. Therefore much of the intentionally-diffused image falls outside the area of the bundle of fibers, resulting in gross optical inefficiency from this cause alone. As a result, the optical efficiency of most typical prior art illuminators is less than 10%.
One reason prior art fiber optics illuminators, even with one or more of the forgoing heat removal methods, continue to overheat the fiber ends is that the fibers are terminated in a bundle that is supported in a rubber compression bushing, much like a rubber chemical bottle stopper with a hole in the center. The bushings in this widely-used practice hold the fibers centered in the aperture of the illuminator, but the rubber is a thermal insulator that precludes the heat generated at the fiber ends from being conducted or radiated out of the fiber bundle.
The basic purpose of the present invention is to provide a fiber optics illuminator in which the focussed energy energy falls substantially on the face of the receiving end of the fiber bundle with minimal spillover losses, without excessive filtering losses, and with fiber operating temperatures within the plastic fiber manufacturers' recommendations.